Pedum (bivalve)

Lore & Background

The two shell valves of Pedum are fan-shaped and asymmetrical: the right valve has a deep byssal notch, is inflected, smoother, and is wider and larger than the left valve, which is flattened and possesses radial lines and spines. The scallop's mantle folds and tentacles are part of its visible anatomy; two tentacle types have been observed: a wholly white tentacle and a brown-striped variety. Like other scallops, Pedum possesses eyes.

Pedum occupies a unique niche among scallops by boring into living stony corals, burrowing into their skeleton in a 'nestling' life habit. Phylogenetics suggests that this behavior arose from ancestors which used byssus to attach themselves to a substrate. Depending on the shape of its host, Pedum may embed itself obliquely (in plate-shaped corals) or deeply (in massive corals). To allow for the maintenance of its crevice, Pedum secretes a chemical that erodes the coral's skeleton and tissues, which has been compared to the distantly related Lithophaga mussels. Juveniles initially settle on the surface of a coral colony, slowly embedding themselves and expanding their crevice; during this stage, Shell Width increases faster than Shell Height. As it grows, the right valve firmly attaches to the crevice's walls, allowing the left valve to open and close. The scallop's growth keeps pace with its coral host's own growth rate. Additionally, the pallial tissues of the mantle along with the tentacles attack the coral's tissues immediately surrounding the crevice, which maintains the opening necessary for the scallop and prevents it from being entombed. This action leaves a 'band' or rim surrounding the opening, which displays chemical etch marks when observed under a microscope. To retract, the scallop pulls its strand of byssus.

Reader's Guide

Pedum's significance lies in its unique ecological role as a coral-boring scallop, a lifestyle rare among Pectinidae. The species demonstrates a specialized adaptation where it chemically erodes live coral to create a permanent crevice, while its growth matches that of the host coral, preventing entombment. Notably, Pedum may benefit its coral hosts by hindering predation by the crown-of-thorns starfish (Acanthaster spp.), which are repelled by jets of water from the scallops; Pedum-inhabited corals were left uneaten by the starfish, leaving a remnant region of live coral around the scallops. This species does not seem to tolerate the adverse conditions of marine lakes, where mytilid bivalves and polychaete tube worms occupy its filter-feeding niche. Examination of coral growth rings containing Pedum crevices suggests that the scallops may live for at least 6 years. The species is the only member of the tribe Pedini, underscoring its evolutionary distinctiveness within the scallop family.

Did You Know?

• It secretes a chemical that erodes coral skeleton and tissues, similar to Lithophaga mussels.
• Pedum-inhabited corals were left uneaten by crown-of-thorns starfish.